Al-hg-bi alloy galvanic anode



United States Patent 3,424,666 Al-Hg-Bi ALLOY GALVANIC ANODE John T. Reding, Freeport, and John J. Newport III, Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware N0 Drawing. Filed Apr. 15, 1966, Ser. No. 542,728 US. Cl. 204-197 4 Claims Int. Cl. C231? 13/00 This invention relates to sacrificial galvanic anodes and more particularly is concerned with a novel aluminum based alloy exhibiting high oxidation potential and a useful high electrical output per unit mass of metal consumed, i.e. a high electrochemical equivalent, which is suitable for use in such galvanic anodes.

Theoretically, aluminum should be expected to perform satisfactorily as a galvanic anode because the element aluminum fulfills the two primary requirements for anodes: (1) a high theoretical oxidation potential (1.90 volts versus calomel reference) and (2) a high theoretical electrical output per unit mass of metal consumed (2.98 amp-hours per gram). In actual practice, however, aluminum has not proved to be satisfactory for use in such applications since it does not exhibit these favorable theoretical properties when used as a sacrificial galvanic anode. The presence of the normally passive oxide surface film on the aluminum apparently presents a barrier to the oxidation of the aluminum metal thereby reducing the effective oxidation potential to about 0.7 volt (as measured in closed circuit at either 250 or 1000 milliamperes/square foot in a synthetic sea water electrolyte with a standard saturated KCl calomel cell as reference). At such low operating voltages, no cathodic protection is given to ferrous based structures, for example; therefore the anode exhibits no useful electrical output. By comparison, the actual working potential of magnesium is about 1.5 volt and of zinc is about 1 volt.

It is known in the art to add certain elements such as mercury or bismuth to aluminum in an attempt to provide an aluminum anode of commercial utility. Such additions have not been successful in that no marked increase in oxidation potential along with feasible efficiency has been realized.

It is a principal object of the present invention to provide an aluminum based galvanic anode which exhibits both high oxidation potential and a useful high amperehour output.

It is another object of the present invention to provide a novel aluminum alloy particularly suitable for use as a sacrificial galvanic anode.

It is a further object of the present invention to provide a novel aluminum-mercury-bismuth alloy which gives good performance with respect to potential and current efficiency (electrical output per unit mass of metal consumed) when employed as a galvanic anode. A related object is to provide a novel alloy composition providing a potential intermediate that offered by zinc and magnesium when employed as a sacrificial anode.

These and other objects and advantages readily will become apparent from the detailed description of the invention presented hereinafter.

The present invention comprises a novel aluminum based alloy composition containing small amounts of bismuth and mercury and to galvanic anodes prepared therefrom.

More particularly, the present novel alloy composition comprises from about 0.003 to about 0.04 weight percent mercury, 'from about 0.015 to about 2 weight percent bismuth and balance aluminum, the alloy being further 7 characterized in that the weight proportion of bismuth/ mercury ranges from about /1 to about 50/1.

Patented Jan. 28, 1969 Preferably the alloy comprises from about 0.008 to about 0.03 weight percent mercury, from about 0.1 to about 0.9 weight percent bismuth and balance aluminum, the alloy being further characterized in that the weight proportion of bismuth/mercury ranges from about 12/1 to about 30/1.

Unexpectedly, the present novel compositin when employed as sacrificial galvanic anodes exhibits a satisfactory smooth corrosion pattern, a high oxidation potential of about 1.3 volts (vs. saturated calomel) and a high electrical output per unit mass of metal consumed (high electrical equivalent). These anodes are particularly suitable for applications where an excessive number of zinc anodes would 'be required to obtain a predetermined current; i.e. effective protection over a predetermined period of time, and where magnesium anodes would provide an undesirably high operating potential.

Galvanic anodes can be prepared from the present alloy by use of alloying and casting or fabricating techniques ordinarily employed in the aluminum art. No special metal handling or fabricating operations are required. Conveniently, in preparing cast anodes, the alloy is prepared by adding the mercury component tothe reactant mixture in the form of a bismuth-mercury prealloy.

Aluminum for use in preparing the present novel alloy compositions can be commercial grade (99.5 to 99.9 percent aluminum) metal having normal production introduced impurities associated therewith. If desired, higher purity aluminum (e.g. 99.99 percent purity) can be employed but this purity is not necessary to achieve high potential and high efliciencies in anodes prepared using the alloy composition. The alloying ingredients also can be either of high purity or of commercial grade.

The resulting alloy product is not detrimentally degraded through air oxidation by storage in normal atmospheres.

The following example will serve further to illustrate the present invention but is not meant to limit it thereto.

Example About 800 grams of primary commercial grade aluminum was melted in a graphite crucible positioned Within an electric furnace. When the melt temperature reached about 750 C., addition of a 10:1 'by weight BizHg prealloy and bismuth, in amounts calculated to provide predetermined amounts of bismuth and mercury in the final alloy, were made to the molten aluminum. The resulting mixture was stirred to effect dispersion and solution of the alloying ingredients throughout the melt. After about 5 minutes, the crucible was removed from the furnace and the alloy was cast in a graphite mold into cylindrical specimen about 5 /2 inches long and either about /8 or 1 inch in diameter. The cooling and solidification rate of the castings were controlled such that these simulated the cooling rate experienced in production of commercial, field-sized cast anodes.

The performance of the alloys was evaluted by positioning a specimen of the cast cylinder (as anode) in a one-half gallon glass jar. A steel wire mesh cloth was placed adjacent to the inner wall of the jar as a cathode. Synthetic sea water was used as an electrolyte with about 3 inches of each anode specimen being immersed. The cells were completed with respect to electrical circuitry, a rectifier being employed to maintain a constant direct current through a group of cells connected in series.

The results presented in Table I show the performance at a current density of about 1000 milliamperes per square foot over a 47 day test period of a number of the novel aluminum alloy anodes of the composition 0 of the present invention. These results present data showing both the solution potential and electrical output per unit mass of metal (efficiency) for the anodes tested.

Potential readings (vs. saturated calomel) were taken several times during the test period. The current efficiency was determined by weighing the anode before and after testing and comparing the Weight loss with the theoretical calculated weight loss.

As is evident from the results presented in Table I, the novel compositions of the present invention exhibit both a useful working potential and a high electrical output. Because of these desirable characterictics these alloys not only are suitable for use as sacrificial galvanic anodes for cathodic protection but also find use in other applications such as, for example, galvanic pigments in paints, galvanic anode materials for primary batteries, sacrificial galvanic coatings for sheet steel and other metals cathodic to aluminum. Additionally, these compositions find utility as an active ingredient in flares, for use in chemical reductions and in the preparation of aluminum alkyls.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. An aluminum alloy having a high oxidation potential and a high electrical equivalent which comprises:

from about 0.003 to about 0.04 weight percent mercury,

from about 0.015 to about 2 weight percent bismuth,

and

balance aluminum,

said alloy being further characterized in that the weight 4 proportion of bismuth/mercury ranges from about 5/1 to about /1.

2. The alloy as defined in claim 1 and comprising from about 0.008 to about 0.03 weight percent mercury, from about 0.1 to about 0.9 weight percent bismuth, balance aluminum, and said alloy being further characterized in that the weight proportion of bismuth/mercury ranges from about 12/1 to about 30/1.

3. An aluminum based sacrificial galvanic anode having a useful oxidation potential and a high electrical equivalent which comprises:

a cast anode structure, said structure comprising,

from about 0.003 to about 0.04 weight percent mercury,

from about 0.015 to about 2 weight percent bismuth,

and

balance aluminum,

said alloy being further characterized in that the weight proportion of bismuth/mercury ranges from about 5/1 to about 50/1.

4. The aluminum based sacrificial anode as defined in claim 3 wherein the cast anode structure comprises from about 0.008 to about 0.03 weight percent mercury, from about 0.1 to about 0.9 Weight percent bismuth, balance aluminum, and said alloy being further characterized in that the weight proportion of bismuth/mercury ranges from about 12/1 to about 30/1.

References Cited UNITED STATES PATENTS 2,565,544 8/1951 Brown 204-l97 2,758,082 8/1956 Rohrrnan 204197 3,186,836 6/1965 Pryor et a1. l38 3,257,201 6/1966 Raclot 75146 3,343,948 9/1967 Raclot 75l38 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. Cl. X.R. 

1. AN ALUMINUM ALLOY HAVING A HIGH OXIDATION POTENTIAL AND A HIGH ELECTRICAL EQUIVALENT WHICH COMPRISES: FROM ABOUT 0.003 TO ABOUT 0.04 WEIGHT PERCENT MERCURY, FROM ABOUT 0.015 TO ABOUT 2 WEIGHT PERCENT BISMUTH, AND BALANCE ALUMINUM, SAID ALLOY BEING FURTHER CHARACTERIZED IN THAT THE WEIGHT PROPORTION OF BISMUTH/MERCURY RANGES FROM ABOUT 5/1 TO ABOUT 50/1. 